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SWITCHING POWER SUPPLIES, OR IíM NEVER TURNING MY COMPUTER ON AGAIN

Nostalgia

Remember the TV ad that went something like, "The worst thing you can do is start your car"? Every time you push your computerís power button, something akin to starting your car takes place. In the case of the combustion engine, cold, thick oil sitting at the bottom of the drain pan canít suddenly shoot into the engine block and properly lubricate all internal parts. The time lag when waiting for the pressure to build does increase wear in the cylinder walls.

Those cheap, nasty switching power supplies, some 235 watt, some 300 watt or more have a similar effect on your motherboard (MB) every time the power button is pushed. If they were of the "linear" configuration, the poor old MB wouldnít get such a shock of 5, -5, +12, and Ė12 (and others) volts. You see, switching power supplies are some of the weakest components, if not the weakest, in that precious little box that is such a source of frustration and entertainment.

If youíre old enough to remember the days of the 286 machine, you may recall the huge, heavy power supply that resided within (and the real ball bearing fan) and an unusually well constructed case. If it ever fell on your foot, an emergency room had better be close by. Many of those old bomb computers came with linear power supplies. Some of which still work after 15 or more years, while Joe Blowís 250 watt, 3 year old generic unit "popped" and caused a frazzled user to run down to the nearest retail store to inquire about another small metallic 250 watt wonder.

I Donít Get It

I still canít understand why we will shell out 500 smackers for the latest processor or even 250 George Washingtonís for the most recently touted "Gee-Whiz-Force" video cards, but when it comes to the power supply, a very crucial part, we canít bring ourselves to fork over much more than ten lousy bucks. Thatís right, about 10 dollars is what we seem to commonly spend on supplies in the 200-250 watt range, when purchased with a corresponding case. And we are amazed that AMD has a recommended list for their Athlon processors.

Realistically, no significant competition exists in the market for computer power supplies other than what we have become used to. You think that the sub-$500 buyer would spring for that 15 year warranted supply in a system that he/she will throw away in three years? I think not. But why donít we have high quality linear units for servers and other demanding workstation/gaming environments? Two reasons: (1) zero demand, and (2) high costs. A small, lightweight (read: less shipping charges) 300 watt unit may cost $10 of the $50 total cost, when purchasing any number of AT or ATX style cases. If a similar capacity linear unit would be substituted, the previous figure would jump to around $30-$50. But, you wouldnít believe the advantages a class A, linear power supply would have for your beloved box.

More History

Bulky, linear units were the standard until around 30 years ago, when engineers figured out a way to dramatically reduce the weight and heat dissipation of the current crop of supplies. In the race to make electronic products smaller and smaller, physics appeared to be limiting just how tiny any device could shrink. By accepting a huge amount of garbage in and frantically trying to filter out the AC (alternating current) ripple at the output, heat dissipation and weight could be reduced by as much as 70-90%.

Whatís AC ripple? Take the amount of "dirty" voltage, which will be in an AC form and divide that into the absolute value of the DC (direct current) voltage. That result represents a percentage or ratio of the bad stuff (AC) and the good stuff (DC). For example, say the AC hash measures 0.030 volts (30mV) and the DC component is 12 volts. Dividing 0.03 by 12 gives 0.0025 or 0.25% of the output is garbage (dirty). More importantly, this AC garbage has a frequency, sometimes in the worst case scenarios, several frequencies. Ever hear a whine when listening to your car radio that follows the engine rpm? That is usually an indication that your voltage regulators or filter capacitors/inductors are on vacation. Now imagine youíve just spent $500 on RDRAM and you just canít seem to get your system stable at 700Mhz. Couldnít be the power supply, nah, Iíll RMA the motherboardÖ

Principles Of A Switching Supply

 

No matter what country where you live, AC comes out of the wall socket. Maybe itís at 100 volts or 120 volts or 240 volts, but itís AC. Hereís a crappy hand drawn picture of what AC looks like on an oscilloscope.

Actually, the quality or smoothness of your AC depends on your supplier. Some countries are well known for their "dirty" power. A DC curve isnít really a curve, just a flat line.

Some engineer, probably with a mathematics background, figured out that if you chop or slice a very small piece of that swoopy AC line, and fiddle with it, a rough approximation of DC begins to take shape. As demand for greater current (amperage) increases, it becomes more difficult to grab a small piece of relatively flat AC, so the snippet nabbed becomes even smaller. Sounds like the frequency of chopping the AC, must be increased, which is exactly what happens. This means that there exists, inside your power supply, a low frequency generator. These modern switching supplies may not be all that low as their frequencies can top out in the Megahertz range. Back to the radio analogy. Try turning on an AM radio near your box and see what happens. You will probably hear a squeal or squawking that makes listening unbearable. Moving the antenna away from the computer reduces the noise. During high current periods, the obnoxious noises can vary as much as an octave or more.

Principles Of A Linear Supply

Linear implies that for every change, a proportional change occurs. A disproportional change may be likened to spilling Hawaiian Punch on the carpet, then shooting the dog for sniffing it.Linear power supplies are much more reasonable or "forgiving" as the case may be. Linear supplies donít necessarily have to include the dreaded feedback loop, which is the main culprit that causes squealing in the output. One example of a class A, non-feedback linear power supply (my favorite to build) is shown below.

Seems so simple, right? To contrast, take a look inside the switching wonder next time you pull your case cover off. Remember to disconnect any power to the supply! What isnít shown here is the large and heavy power transformer (which just two inductors) and the full wave rectifier made with 4 diodes. The switching unit has the advantage of not using a large transformer or huge filter capacitors, as C would denote.

From left to right, the raw rectified AC enters and is filtered with C. The value of C for a 300 watt supply could easily be in the 10,000 to 33,000 micro Farad (uF) range and that would be a rather large and expensive piece. Next, a small fraction of the current, say 1/100th of the total, flows through the resistor R. This current feeds a Zener Diode, denoted by D. The Zener Diode sets up a constant voltage (i.e. 12 volts) and also rejects various AC ripple components. This base voltage feeds the base input of the NPN pass transistor Q and thusly, a regulated voltage spits out of the node labeled DC OUT. Notice that there is no possibly way to take a small portion of the output and feed it back into the input. Hence, this circuit has zero feedback, and cannot squeal. Unlike the switching unit, when this circuit gets loaded with a strong power drain, the ripple merely increases, not a switching frequency in the megahertz range. Lastly, when this circuit powers up, there exists a delay (short, but itís there) instead of WHAM! with the switching supply.

Nifty, So Now What Can I Do?

When purchasing a computer power supply, pay particular attention to the overall power rating. Switching supplies donít like to be run anywhere near their maximum limits, so leave plenty of room. I would venture to guess that most custom builders should use no less than a 235 watt supply. I recently opened up a Hewlett Packard Pavilion series and was shocked to find a wimpy 110 watt unit struggling along inside its incredibly small case. If the owner ever decides to add another hard drive, CD-R or a real AGP video card (instead of 2MB shared RAM type), heís gonna be out of luck. If youíre the SCSI maniac type and have a hot-rod 3D machine, no less than 300 watts is the order of the day. The last time I checked the price difference between 235 watts and 300 watts was a measly 10 bucks. If youíre already spending $1000 or more, donít be cheap with the power supply! Billís done with his rant now, get some sleep.

William Yaple
Bill@TargetPC.Com
99/11/01
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